The field of this invention is electrical power converters and more particularly to such converters capable of converting an unregulated dc voltage to a variable and regulated dc voltage. Prior art converters of these types have included one or more full or half bridge choppers comprising four switching transistors which are rendered conducting in pairs to periodically reverse the current through a transformer primary and thus produce a secondary voltage which is then rectified to produce the output voltage. The duty cycle of the transistors may be controlled by a feedback circuit to regulate the output voltage. A slight delay in the switching off of any of these transistors, for example caused by stored charge in the transistor collector-to-emitter junction leads to a situation known as "switchthrough", wherein two series-connected transistors will short the input voltage, which can lead to destructive current flow. To limit these short-circuit currents, current-limiting inductors have been used in series with the input voltage supply. These inductors however produce a high flyback voltage each time the current therethrough is switched off and this occurs at least once for every chopper cycle. This flyback voltage can cause transistor and inductor failure because of its high amplitude, and it can result in inefficiency especially in high frequency systems since the energy stored in the inductor's magnetic field is normally dissipated at least once during each chopper cycle. A Bell Telephone Lab circuit with a single choke in series with the input converter chopper negated the effects of transistor switchthrough and also provided current limiting for transient load shorts. That circuit however has the disadvantage that a flyback voltage proportional to L di/dt appears on top of the chopper dc output. To prevent these large flyback voltages from causing the chopper transistors to go into first avalanche, the choke must be shunted by a dissapative snubber.
A more efficient means of clamping the flyback voltage is to use an energy recovery circuit that utilizes the primary of a transformer as a current-limiting inductor with one terminal of the secondary thereof having a diode connected to the B+ or converter input voltage in such a polarity that the flyback voltage in excess of twice B+ is fedback to the B+ sink. In some applications a flyback voltage of twice B+ cannot be tolerated.
Prior art chopper switching regulators (or pulsewidth regulators) vary the output voltage by changing the duty cycle or "on" time of a switch which feeds the load through a transformer. During the "off" time of the switch the load is thus isolated from the remainder of the circuitry. This situation results in two disadvantages, (1) the I.sup.2 xR loss associated with the effective series impedance of the switch increases as the inverse of the deviation. This is undesirable when high efficiency is required over a large range of load conditions, as required for example by many space programs; (2) immediately following the opening of the switch, the energy stored in the output transformer generates a voltage transient across the switch which is proportional to the switching current times the transitional open-circuit inductance of the transformer. This causes both first and second avalanche of the switching transistors. Since the transition from leakage to open-circuit inductance is reflected during this period, extensive snubber networks must be used to correct for the large current phase lag so generated. These conditions make for poor reliability and reduce the converter efficiency.
In contrast, in the present invention, the converter utilizes two or more current-fed square wave choppers of variable relative phase, with the outputs thereof combined by series-connected transformer secondary windings. Thus the peak current is little more than the average current over the full 180 degrees of deviation. One of the two choppers is always connected to the load and thus the high open-circuit inductance of the transformer windings is never present. This circuitry eliminates the aforementioned disadvantages of the prior art.